Organic electroluminescent (EL) devices or organic light-emitting devices (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. demonstrated highly efficient OLEDs using such a layer structure in “Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987) and in commonly assigned U.S. Pat. No. 4,769,292. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610(1989). The LEL commonly consists of a host material doped with a guest material. These three-layer structures are denoted as HTL/LEL/ETL. Further, there are other multilayer OLEDs that contain additional functional layers, such as a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or an electron-blocking layer (EBL), and/or a hole-blocking layer (HBL) in the devices. At the same time, many different types of EL materials are also synthesized and used in OLEDs. These new structures and new materials have further resulted in improved device performance.
In a full color OLED display, there are at least three primary colors of emission, i.e. red, green, and blue emission. Currently, red and green OLEDs have better performance than blue OLEDs. It is difficult to achieve both high luminous efficiency and good operational lifetime in the blue OLED. Therefore, improving the performance of the blue OLED will have a large impact on the applications of a full color OLED display. There are several ways to improve the blue OLED performance through material selection, device structure modification, etc. For examples, Shi et al. in “Anthracene Derivatives for Stable Blue-Emitting Organic Electroluminescence Devices”, Applied Physics Letters, 80, 3201 (2002), and Hosokawa et al. in U.S. Patent Application 2003/0077480 A1, both achieved improved operational stability of blue emission by selecting proper materials. Other new methods for the improvement of the blue OLED performance are certainly necessary.
A conventional blue OLED, such as that using 2-(1,1-dimethyethyl)-9,10-bis(2-naphthalenyl) anthracene (TBADN) doped with 2,5,8,11-tetra-t-butylperylene (TBP), denoted as TBADN:TBP, as a LEL is shown in FIG. 1A, wherein OLED 100 includes an anode 120, a HTL 132, a LEL 134, an ETL 138, and a cathode 140. This device is externally connected to a voltage/current source 150 through electrical conductors 160. FIG. 1B shows a corresponding energy band diagram of the OLED 100 in FIG. 1A (in a flat band condition). The dotted lines in LEL 134 are the electron energy levels of the dopant material. In FIG. 1B, the ionization potential of ETL 138 (Ip(ETL)) is equal to or less than that of the LEL 134 (Ip(LEL)), and the electron energy band gap of ETL 138 (Eg(ETL)) is narrower than that of the emissive dopant material (Eg(dopant)) in LEL 134. This is the case when tris(8-hydroxyquinoline) aluminum (Alq) is used as the ETL 138. Since the Eg(ETL) is narrower than the Eg(dopant) in this case, some excitons formed in LEL 134 can diffuse into ETL 138 to produce green emission from the ETL 138 causing impure color emission from the device. In addition, since there is no energy barrier to hinder holes moving into the ETL 138 at the interface of LEL/ETL, the hole-electron recombination rate in the LEL 134 is relatively low resulting in low luminous efficiency.
The aforementioned electron energy band gap (Eg) is the energy difference between the ionization potential (Ip) and the electron affinity (Ea) of the film, or the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the film. Practically, the Eg of organic thin films can be adopted using an optical energy band gap of the films, and the Ea of organic films can be calculated by subtracting the Eg from the Ip of the films. The Ip of organic thin films can be measured using an ultraviolet photoelectron spectroscopy, and the Eg of organic films can be measured using a UV-Vis absorption spectrometer.
FIG. 1C shows another corresponding energy band diagram of the OLED in FIG. 1A, wherein Ip(ETL) of ETL 138 is greater than Ip(LEL) and Eg(ETL) is equal to or a wider than Eg(dopant). This is the case when a wide band gap material, such as 4,7-diphenyl-1,10-phenanthroline (Bphen) or 2,9-dimethyl4,7-diphenyl-1,10-phenanthroline (BCP), is used as the ETL 138. In this case, pure blue emission may be achieved. However, we found that although the energy barrier at the interface of LEL/ETL can hinder holes moving into the ETL 138 and increase the luminous efficiency, the accumulated holes and electrons at this interface will shorten the operational lifetime. In order to obtain pure color emission, Toguchi et al. in U.S. Pat. No. 6,565,993 B2 inserted an intermediate layer, IML, between the LEL and the ETL satisfying the relation of Ip(LEL)<Ip(IML)<Ip(ETL). Although there were no lifetime data demonstrated in the patent, we believe that the energy barrier existing at the interface of LEL/IML can reduce the lifetime.
In some cases, for example, as disclosed by Fujita et al. in U.S. Pat. No. 6,566,807 B1, an HBL with both an ionization potential (Ip(HBL)) greater than Ip(LEL) and an electron energy band gap(Eg(HBL)) greater than Eg(dopant) can be inserted between the LEL and the ETL to prevent holes from escaping to the ETL and to increase the hole-electron recombination rate (if lp(HBL)=Ip(LEL), it is unlikely that there would be a noticeable hole-blocking effect). Such a device structure is shown in FIG. 2A, wherein OLED 200 has one more layer, i.e. HBL 236, than OLED 100 in FIG. 1A. According to the energy band diagram shown in FIG. 2B, holes can be accumulated at the interface of LEL/HBL to increase the luminous efficiency. In this case, excitons that form in the LEL and then diffuse into the HBL will not cause impure color emission in the HBL, regardless if the Ip(ETL) is less than, equal to, or greater than Ip(LEL). However, similar to OLED 100 having an energy band diagram of FIG. 1C, the blue OLEDs with a hole-blocking layer also have a shortened operational lifetime.